The use of the noble metals platinum, palladium, iridium, and ruthenium to oxidize harmful combustible constituents such as carbon monoxide and hydrogen present in automobile exhaust gases is well known. For example, U.S. Pat. No. 2,742,437 discloses a catalytic structure suitable for use in automotive exhaust systems to oxidize harmful exhaust constituents comprising an impervious support structure having an intermediate metal oxide support coating and a catalytic coating of platinum, ruthenium, iridium, or palladium. The noble metal catalytic coating is effective to oxidize at least a part of the combustible constituents present in the exhaust gases passing through the passages in the structure to carbon dioxide and water.
It is also known to use noble metals to reduce the oxides of nitrogen present in industrial waste gases. U.S. Pat. No. 3,032,387, for example, describes the use of platinum, ruthenium, and palladium as catalysts to promote a reaction between NO.sub.2 and a fuel gas to produce NO and a further reaction between NO and NH.sub.3 to produce nitrogen and water.
Oxides of nitrogen in automobile exhaust gases present a special problem because substantially complete elimination must be accomplished without the introduction into the atmosphere of harmful reduction by-products and, preferably, without the need for the use of fuel gases or ammonia to aid in the neutralization process. Thus, while the use of pure noble metals to catalytically promote the reduction of the oxides of nitrogen is known, most prior art processes either require the use of ammonia and fuel gases to aid in the reduction process or produce harmful gases such as ammonia as by-products of the catalytic reduction.
The removal of nitrogen oxide pollutants from automobile exhaust gases should preferably be accomplished with a small self-contained unit requiring no inputs of neutralizing gases and producing no harmful reaction by-products. Physical and thermal stability are further requirements of such a unit. Repeated physical shock, thermal cycling and sustained operation at temperatures perhaps as high as 1600.degree. F. may be encountered. Thus, the catalyst system employed must be able to retain a useful degree of catalytic activity over a broad range of temperatures for extended periods of service, and the components supporting the catalyst must not be degraded by high temperatures or thermal shock.
It is one object of the present invention to provide a process for catalytically treating automotive exhaust gases to remove the nitrogen oxides present therein without the addition of neutralizing fuels or gases and without producing objectionable quantities of reaction by-products such as ammonia.
It is a further object of the present invention to provide supported mixed noble metal catalyst systems which are useful in the process of catalytically treating exhaust gases according to the present invention.
It is another object of the present invention to provide supported mixed noble metal catalyst systems which simultaneously promote not only the reduction of nitrogen oxides in exhaust gases, but also simultaneously promote the oxidation of carbon monoxide and unburned hydrocarbon constituents present therein.
Other objects and advantages of the invention will become apparent from the following description and examples thereof.